The present invention relates generally to differential signal acquisition probes and more particularly to a differential signal acquisition probe having individually retractable double cushioned probing tips and electrical over stress (EOS) and electrostatic discharge (ESD) protection capabilities.
Ultra high speed sampling heads used in time domain reflectometry typically dictate extremely low capacitances. This introduces unique problems. Sampling devices are much more sensitive to static discharge residing on a device under test. The small geometry of the sampling diodes in the sampling head often dictate low breakdown voltages. The low parasitic capacitance at the sampling head input means that for a given device under test (DUT) static discharge, there will be a higher transient voltage at the sampler input because of the reduced charge sharing effect. It is therefore important to neutralize any static charge on the device under test before the sampling head input is coupled to the device under test.
U.S. Pat. No. 6,734,689 describes a measurement probe providing signal control for an EOS/ESD protection control module. The measurement probe has a spring loaded coaxial probe assembly and a pressure sensor that work in combination to provide an activation signal to the control module. The spring loaded coaxial cable assembly and pressure sensor are disposed in a probe housing. The spring loaded coaxial probe assembly has a semi-rigid coaxial cable with one end forming a probing tip and the other end having a threaded connector. A flexible coaxial cable is connected to the threaded connector and to the control module. A compression spring is positioned over the semi-rigid coaxial cable with one end secured to the semi-rigid coaxial cable and the other end engaging the probe housing. The compression spring is pre-loaded to apply an initial force to the spring loaded coaxial probe assembly as shown graphically in FIG. 1. FIG. 1 shows the forces applied to the probing tip of the spring loaded coaxial probe assembly during use where “F” is the force applied to the probing tip, k1 is the spring constant, and ΔX is the displacement of the spring from its equilibrium position. The pre-loading of the compression spring generates an initial force F1 on the coaxial probe assembly. The pressure sensor has one electrical contact attached to the outer shielding conductor of the semi-rigid coaxial cable which is connected to electrical ground via the flexible coaxial cable. The other pressure sensor electrical contact is mounted to the probe housing. An electrical conductor electrically couples the pressure sensor to the control module.
The control modules provides a ground circuit path for the signal conductor of the measurement probe when the activation signal is absent. When the probing tip makes contact with the device under test, any static electricity on the DUT is coupled via the signal conductor to ground via the control module. As downward pressure is applied to probe housing, the coaxial probe assembly retracts into the probe housing. The compression spring exerts increasing pressure on the coaxial probe assembly following Hook's Law of F=k1 ΔX where k1 is the spring constant. Continued downward pressure applied to the probe housing results in the pressure sensor contacts making contact. This results in the pressure sensor passing an activation signal which controls switching circuitry in the control module that removes a ground connection on the signal conductor of the measurement probe. Since the pressure sensor contacts are fixed to the semi-rigid coaxial cable and the probe housing, any continued downward pressure on the probe housing transfers the forces to the pressure sensor and the coaxial probe assembly as represented in FIG. 1 by the vertical force line. The excess forces on the pressure sensor and the coaxial probe assembly may result in damage to the pressure sensor or the coaxial probe assembly.
Another example of a signal acquisition probe for TDR applications is the CP400-04, manufactured by Candox System of Japan. This probe has a metal housing in which an insulated signal conductor is disposed. The metal housing has a threaded connector at one end for connecting a signal cable. The other end of the housing has apertures for receiving spring action pogo pins. One pogo pin is coupled to the insulated signal conductor and the other pogo pins are connected to the metal housing. Non-conductive stops are placed on the end of the housing with the pogo pins to limit the travel of the movable contacts to prevent damage to the pogo pins. Assuming that the retractable portion of the pogo pin is pre-loaded, the force on the retractable portion of the pogo pins is similar to that of FIG. 1. The pre-loaded retractable portion has an initial force F1. Downward pressure on the metal housing creates an increasing force on the retractable portion of the pogo pin as represented by line k1. As lone as the probe is vertical to the DUT the electrical characteristic of signal line and the ground lines are the same. If the probe is not normal to the DUT, the one or more of the pogo pins will not be retracted to the same extent as the other and the electrical characteristic of the signal or ground lines will change leading to inaccurate measurements. Further, this probe does not have EOS/ESD protection.
U.S. Pat. No. 6,704,670 describes a wideband measurement probe for single ended and differential active probing of devices under test. The measurement probe includes at least a first typically cylindrical probe barrel. The probe barrel is constructed of an electrically conductive material and extends partially outside of a probe unit housing. A probe barrel nose cone is attached to the exposed probe barrel. The probe nose cone is generally conical in form and made of an insulating material. The longitudinal axis of the probe barrel nose cone extends from the probe barrel at an offset angle from the longitudinal axis of the probe barrel. A typically cylindrical shaped probe tip extends partially out of the end of the probe barrel nose cone and is make of an electrically conductive material. A probe cable having an outer shielding conductor and a central signal conductor is connected to the probe barrel and the probe tip with the outer shielding conductor being connected to the probe barrel and the signal conductor being connected to the probe tip. An elastic compressible element engages the probe barrel and the probe unit housing allowing movement of the probe barrel into and out of the probe unit housing. For a single ended measurement probe, a retractable ground tip is attached to the probe barrel. For a differential measurement probe, two probe barrel and probe nose cone assemblies are positioned side by side in a probe unit housing. Individual elastic compressible elements are provided for each assembly. Individual coaxial cables are attached to each assembly.
The forces exerted on the probe barrel and probe nose cone assemblies are comparable to forces shown in FIG. 1. Assuming that the elastic compressible element or elements are pre-loaded, there in an initial force on the assembly or assemblies as represented by the force F1. Downward force on the probe unit housing exerts an increasing force on the assembly or assemblies until the elastic compressible element or elements are completely compressed or the assembly or assemblies engage a fixed stop as represented by the sloping line k1. Continued downward pressure on the probe unit housing transfers forces to the assembly or assemblies as represented by the vertical force line. The above described measurement probe is used for measuring signal from a device under test. As such, the probe has passive input circuitry that lessens the need for EOS/ESD protection. Therefore, these probes do not ground the signal input to discharge electrostatic voltages on the device under test.
What is needed is a differential measurement probe having EOS/ESD protection capabilities. The differential measurement probe needs to discharge static voltages on a device under test prior to the signal channels of the differential measurement probe being coupled to a sampling head. Further, the differential measurement probe should provide an indication to a user that adequate pressure has been applied to the probe so as to prevent damage to the probe.